Calculation electric short circuit of single-phase and three-phase

With this calculator you can know the short-circuit level of single-phase or three-phase electric transformer.

In addition to the calculation is also the  formula that is used for the calculation of short circuit , we explain  how to calculate the short circuit level in 1 step , we list some  calculation examples  and present the  table with the most common short circuit levels .

Formula for calculating single-phase and three-phase short circuits of the transformers (kA):

formula to calculate single-phase and three-phase short circuit

How to calculate the short-circuit level of a transformer in 1 single step:

How to calculate the short-circuit level of a transformer in 1 single step

Step 1:

To calculate the short-circuit level of a transformer, the power must be divided between the voltage, multiplied by the root of three and the impedance of the transformer.

Example : A 75000VA transformer has a low voltage voltage of 220V and an impedance of 3.5%, to find the short level you must divide 75000 / (220 * √3 * 0.035) which will result in: 5623kA.

Note: 35% in decimals is 0.035

Examples of calculation of short circuit levels:

Example 1:

A three-phase transformer of a medium voltage substation has a power of 630000VA, a primary voltage of 13200V and a secondary voltage of 480V, with an impedance of 5%, which short-circuit level has the transformer at low voltage (secondary side).

Rta: // To find the answer you must multiply the voltage by root of three and the impedance, in the following way: 480x√3 × 0,05 = 41,5, then you must divide 630000VA between the previous result 41,5, which will give as a result, 630000VA / 41.5 = 15155kA.

Example 2:

The transformer of an industrial plant is 500,000VA three phase, with a voltage of 4160V in the secondary and an impedance of 5%, which will be the short level of this transformer.

RTA: // To know the short level, the VA must be divided between the voltage multiplication, the root of three and the impedance, as follows: 500000VA / (4160Vx√3 × 0.05) = 1387kA.

Example 3:

The transformer of a small building is 25000VA single-phase, with a voltage of 240V and an impedance of 3%, which will be the short-circuit level of the transformer.

Rta: // To find the answer you should only replace the variables in the short circuit level calculator and it will automatically give the answer of: 3472kVA.

Typical percentage impedance values:

The percentage impedance of a transformer is a measured value that is printed on the nameplate and is actually a voltage measurement.

It is a test that manufacturers do for power distribution transformers and is used in the calculation of the fault current. This is important for coordinating overcurrent protection devices (OCPD), short circuit analysis, harmonic analysis and electric arc studies.

If a transformer has an impedance of 6.33%, it would require 6.33% of the primary input voltage to generate 100% of the rated current in the secondary windings when a fault occurs in the worst case.

In electrical distribution systems, the worst failure is when a low impedance metal bar cuts through the lines and is called bolted failure.

Now, if 100% of the voltage is applied to the primary input, then approximately (100 / 6.33 = 15.8x nominal current), it would flow in the secondary winding under a fault in the worst condition. This is the maximum short-circuit current that we would have in the system.

Transformer size (kVA)Typical values ​​of% Z
 0-150 Below 4%
 Above 10kVA9.5%

Table of short circuit levels (Voltage 220v – 3Ø):

VAImpedance (%)kA

Note: The short circuit levels presented in the above table are for reference and should not be used to size any type of electrical system.

To dimension the electrical systems, the exact plate data of the transformer manufacturer that will be used in the electrical installation must be used.

How to use the short-digit level calculator:

The first thing to enter is the transformer VA then the number of phases, then the voltage and finally the most important value, the impedance of the transformer, you can find reference values ​​in the impedance table.

Qualify short circuit calculator:  [kkstarratings]